The present invention relates to optical and to analytical components comprising embedded microchannels. The present invention also relates to the method of producing such optical and analytical components. The present invention also relates to the use of such optical and analytical components.
Optical components comprising micron or submicron surface structures are widespread in use. The optical field related to such components is the field of diffractive optics. Among different products one of the most famous representative is the compact disc which is roughly speaking a grating structure in a plastic substrate coated with a metal. Other examples are antireflection gratings, grating couplers and wire grid polarizers a well as the micro-electro-mechanical systems and resonant grating devices.
Many of these systems need to be coated either in order to enhance their optical performance or in order to electrically contact the surface or in order to protect the structures. Among the coatings which are typically used in such kind of structures there are four different classes:                conserving coatings: coatings which need to cover all parts of the surface with a thin, homogeneous layer, mainly conserving the surface structure (see FIG. 1a).        filling coatings: coatings which need to fill the structures completely. This class of coating can be subdivided further in coatings where the surface profile needs to be reproduced on the coating surface (see FIG. 1b left side) and coatings where the structure needs to be smeared out (see FIG. 1b right side).        selective coatings: only parts of the structure need to be coated. Coating of other parts needs to be avoided (see FIG. 1c).        cover layers: coatings which cover only the structure without penetrating the valleys, i.e. burying or embedding the structures while leaving the index distribution within the structures mainly unchanged (see FIG. 1d).        
Conserving coatings have the goal to at least substantially conserve the surface structure. The coating needs to be of principally homogeneous thickness independent of the orientation of the features of the surface. This can be realized for example with chemical vapor deposition (CVD) methods, where the surface to be coated is chemically activated and a chemical reaction leads to deposition of material on every part of the substrate in a homogeneous way.
In order to realize filling coatings, chemical vapor deposition can be used as well. However, since the deposition of the particles strongly depends on the flux of the reactive gas used and the small structures create turbulences, as well as flux inhomogeneities, as a consequence inhomogeneities in coating thicknesses may appear. In addition, since the deposition rate (defined as number of particles deposited per time unit) is proportional to the surface presented in an area, this type of coating technique tends to smear out the structure very efficiently. If the structure is to be reproduced in the coating (see FIG. 1b), this kind of filling technique cannot be used.
Other techniques such as sputtering can also be used to fill the structures. However especially for structures with small feature sizes and a high aspect ratio (aspect ratio=the depth of the structure related to the minimum feature size), this often is quite difficult, since self attenuating shadowing effects lead to overhanging structures and to inhomogeneous thicknesses of the coating. Note that this is a problem well known with coatings for semiconductor devices. As described in DE 197 02 388 the sputtering process for semiconductor devices with small feature sizes exactly shows the aforementioned disadvantages.
In order to realize selective coatings evaporation can be done from a more or less distant point source. If the process is realized in such a way that the mean free path of the coating particles is larger than the distance from the source to the substrate to be coated, the coating will be a directional process, thus defining the direction of the particle movement. If the direction is parallel to the normal of the substrate almost no particles will be deposited on the vertical interfaces of the structures.
Sometimes the surface structures need to be conserved and mainly the vertical surfaces need to be coated. In this case a collimating device can be used. Then even sputter sources are applicable, as disclosed in U.S. Pat. No. 6,210,540. Here a coating mask-blocks out the particle flux propagating in directions parallel to the normal of the substrate mechanically. The coating zone is restricted to areas where the particle flux is at an oblique incidence angle with respect to the normal of the substrate to be coated.
Note that in this case a high percentage of the coating material is deposited on the mask and therefore lost for the coating on the substrate. This results in low deposition rates as well as in low efficiency for sputter target use.
Sometimes tilted directional selective coatings in combination with the shadowing effects are used to create etching mask on the top of a structure, covering only the upper parts and leaving the lower parts of the structure unprotected and open for a following etching procedure. This as well can be done either with point sources and sufficient mean free paths of the coating particles or with collimating masks.
For cover layers the situation is quite different. These layers are mainly used for protecting the underlying structures. None of the aforementioned methods can be used without major modifications leading to additional production costs. State of the art teaches that such structures are needed for example for embedded wire grid polarizers as disclosed in U.S. Pat. No. 6,288,840. However nothing is said how such cover sheets could be realized. In today's applications it is common to protect the structures using thin glass cover sheets, mechanically cemented to the structures. Note that in many cases no adhesive can be used in the structure region since this would fill the structures and influence the optical performance of the device. In addition, in order to fully protect the small features, such devices very often have to be sealed. This is quite expensive to realize and very often the disadvantageous optical influence of the glass sheet cannot be avoided.
It is clear from the description above that according to procedures of the described prior art it is not known how to practicably cover the microstructures without almost completely filling the grooves in the structure. For example with wire grid polarizers as discussed in U.S. Pat. No. 6,288,840 (hereinafter referred to as '840), it is necessary that the grooves between the metal rods form hollow spaces, since an increase of the index of refraction in the grating grooves directly affects the performance of the component. However as is discussed in '840 it is advantageous to use an embedded wire grid polarizer. As described in '840 such a polarizer comprises a first layer, a second layer and an array of parallel, elongated, spaced apart elements, sandwiched between the first and the second layers. The inventors mention that in a preferred embodiment the material in the grooves will be air or vacuum, but for reasons of practicality other materials may be used. It is concluded that the realization of embedded groove structures is not practicable.